Study On Deformation Characteristics Of Shale Self-propped Fractures Under Normal Stress

Shale gas has gradually become the focus of unconventional energy exploration and development in China.Due to the tight lithology and strong heterogeneity of shale reservoirs,hydraulic fracturing is an essential technical means to ensure the efficient development of shale reservoirs.Self-propped fracture is an important oil and gas flow channel formed in the process of shale gas hydraulic fracturing.However,self-propped fracture is highly sensitive under high closed stress,which often fails to provide continuous and effective conductivity for fracture network.The existing research methods of self-propped fractures are mainly experimental research.In the case of lack of core,the experimental research is limited by many factors.At the same time,only through the experimental means,it is difficult to study the influence of different closure stress,rock mechanical properties and surface topography and other factors on the law of fracture deformation,and it is not able to effectively analyze the deformation characteristics of self-propped fractures under normal stress.Therefore,in this paper,Abaqus finite element numerical simulation method was used to carry out relevant research,focusing on the rough surface characteristics of self-propped fractures and the compression deformation of fractures.First,3d laser scanning was used to obtain the surface data of rough fractures in LongMaXi formation shale of south Sichuan,and Kriging interpolation method was used to reconstruct the data of fractures.Statistical theory,fractal theory and JRC roughness coefficient were respectively used to study the roughness characteristics of the reconstructed fracture surface.The self-propped fracture width formed by the shale of this reservoir exhibited a unimodal feature similar to normal distribution and presented a strong fractal feature,with a fractal dimension of 2.4671-2.8795 and a JRC roughness coefficient of 11.22-19.78.Based on the calculation results,the complexity coefficient of rough convex points was proposed,and the self-propped fractures were divided into three typical rough convex points(cubic,V and sinusoidal)fracture models.Secondly,using the idea of piecewise function and hyperbolic approximation to optimize the mole-coulomb classical elastic-plastic yield criterion,the singularity of the mechanical constitutive model in describing the abrupt point of irregular geometry is corrected.On this basis,a UMAT subroutine was developed for the secondary development of Abaqus,which could update the determination of the elastoplastic behavior of convex points.The degree of stress and deformation of four groups of real self-propped cracks was calculated.The reliability of the secondary development subroutine and the simulation method was verified by comparing the experimental test results with the model calculation results.Finally,since the support are analyzed based on the principle of single variable deformation characteristics of cracks under normal stress,surface geometric shape of cracks,and study the level of slippage and rock mechanical parameters(young's modulus,poisson's ratio)on the influence law of fracture closure,analysis when the normal stress is low(0 to 30 mpa),the surface roughness is the main factors influencing the fracture deformation characteristics;When the normal stress is high(>30MPa),the influence of rock mechanics parameters on fracture deformation characteristics is more significant.Since the support under different normal stress is presented in this paper the fracture stress deformation finite element model,based on the secondary development of Abaqus to modify Mohr coulomb criterion,the established model can simulate the real morphology under the support stress deformation characteristics of cracks,and reveals the self-propped crack deformation characteristic under the high closure stress,for deep shale and shale from propped research provides a new train of thought.